Large tractor trucks typically have air-conditioning systems similar to automobiles. The truck engine drives a compressor that compresses refrigerant and delivers it to a condenser. The condenser converts the hot gaseous refrigerant to a liquid refrigerant. The liquid refrigerant flows to an evaporator where it undergoes a pressure drop, converting the liquid refrigerant to a cold gas. An interior blower moves air through the evaporator and into the interior of the vehicle. The condenser is cooled by the main engine fan, which also flows air through the engine radiator.
For heating, a heater coil or element is mounted in the vehicle in communication with the radiator via hoses. A portion of the hot engine coolant flows through the heater element. The interior blower moves air through the heater coil to heat the interior of the vehicle.
Many large trucks have sleeper compartments attached to or part of the cab for allowing the driver to rest. Often, the truck will have a rear unit for heating and cooling the sleeper compartment. The rear unit comprises an evaporator in parallel with the cab evaporator, and a heater element in parallel with the cab heater element. The rear unit uses the same compressor and condenser as the cab unit. In order for the rear heating and cooling system to operate, the operator must run the main truck engine. Often while operating the rear heating and cooling system, the truck is parked. The main engine generates far more power at idle than is needed for operating the rear heating and cooling system, resulting in a waste of fuel.
One approach for heating and cooling a sleeper compartment while the truck is parked is to utilize a 110 volt air conditioning unit to the truck. Some rest stops have service receptacles that enable the operator to connect a power cord to the 110 volt air conditioning unit. Many rest stops, however, do not have such provisions for connecting a vehicle to AC (alternating current) electrical power.
Also, in the prior art, generators have been mounted to large trucks for generating 110–115 volt AC power. An auxiliary engine, normally diesel, is located in a compartment along with an AC generator. The generator powers a 110 volt air conditioning unit mounted to the sleeper compartment. The air conditioning unit has an AC electrical motor that drives the compressor and the fan. For heat, an electrical resistance element is employed. These systems are normally referred to as “gen-sets”.
Another prior art approach, sometimes referred to as an “auxiliary power unit”, utilizes an auxiliary engine in an external compartment to directly drive a motor vehicle type compressor. A condenser and radiator are located in the external compartment. An evaporator, heater element, and DC (direct current) blower are mounted within the sleeper compartment. The compressor delivers refrigerant to the condenser, which in turn delivers condensed refrigerant to the evaporator for cooling the interior of the truck. For heating, a portion of the coolant from the auxiliary engine may be diverted from the radiator to the heater element. In one such system, when the heater is turned on, a valve assembly proportionately reduces the coolant flowing to the auxiliary radiator while increasing the coolant flowing to the heater element.
The prior art auxiliary power unit does not have a generator, rather has an alternator that supplies DC (direct current) voltage. The DC voltage powers the blower for the evaporator and heater element and drives the fan for the condenser and radiator. The alternator of the auxiliary unit also charges the main truck batteries. This type of unit may also have an inverter that converts part of the DC voltage from the auxiliary engine alternator into AC voltage for running AC appliances in the sleeper compartment.